1. Field of the Invention
This invention pertains to means for determining range to an object and more particularly relates to the determination of range to an accuracy of less than two feet (60.96 cm).
2. Description of the Prior Art
Measurement of range electronically involves the determination of the time difference of arrival between an echo signal and a suitable reference. Usually the reference is a transmitted signal which must be marked in some distinguishing manner to establish an initial time at the receiver from which the time of the return of the echo may be determined. This timing mark may be referenced to a transmitted pulse, as in a pulse radar, and the range determined from time of return of the echo pulse. An analog approach to the problem involves starting a ramp at the instant of transmission which establishes t = 0 and terminating the ramp at t = .tau., the time at which the echo is received. The d.c. voltage at the termination of the ramp is held by operational amplifiers and other d.c. stabilizing networks until the beginning of a new period, at which time the ramp is reset. D.C. average voltage over a number of transmission periods, which varies as a function of range, is measured by a d.c. meter. The degree to which the ramp voltage at t = .tau. is maintained by the stabilizing circuits determines the accuracy of the measurement. Another analog technique employs a course range delay generator, such as a multivibrator, and a fine range generator, consisting of one or more precision phase shifters which provide the ultimate time reference in terms of fractions of a cycle of a basic ranging oscillator, to determine the time between the transmitted pulse and reception of its echo. These systems exhibit systematic errors of several yards (meters) even with linearities of one-tenth of one percent.
A digital approach to this problem, capable of achieving range accuracies in the order of one foot (30.48 cm), involves the gating of a 1 GHz oscillator and counting cycles. To count at a 1 GHz rate requires expensive and elaborate circuitry and consequently is not an attractive solution to the problem. The subject invention discloses a coarse range-fine range system with which the accuracy obtainable by digital techniques may be inexpensively achieved. This is accomplished by combining conventional digital techniques with a distributed network approach to provide digital processing accuracy at a saving of an order of magnitude or more in the cost of components.